Microwave Photonics Applications Research Articles

High-input dynamic range and selectivity stimulated Brillouin scattering-based microwave photonic filter utilizing a dual-stage scheme.

A high-input dynamic range and selectivity stimulated Brillouin scattering (SBS)-based microwave photonic filter (MPF) is proposed and experimentally demonstrated. By utilizing a pump-splitting dual-stage scheme to mitigate the gain saturation, an input dynamic range of >40 dB and a selectivity of >49 dB are achieved simultaneously for the first time, to the best of our knowledge. The bandwidth is reconfigurable from 50 MHz to 200 MHz by tailoring the Brillouin pump precisely. During the bandwidth turning, the passband ripple is maintained <1.3 dB, and the 20-dB shape factor is 1.2 for the 200-MHz case. The superior performance makes the proposed SBS-MPF one of the best candidates for microwave photonics applications.

Continuously tunable ultra-thin silicon waveguide optical delay line

As light cannot be stopped or directly stored in any media, optical delay lines are usually used to temporally trap the optical signals. We report a wide-range continuously tunable optical delay line chip consisting of a ring resonator array and a Mach–Zehnder interferometer (MZI) switch array on the 60-nm-thick silicon waveguide platform. The ring delay line provides continuous delay tuning of more than 10 ps with a push–pull differential tuning method. The MZI switchable delay line provides digitally programmable delay tuning with a resolution of 10 ps upon reconfiguration of the MZI switches to establish different optical routing paths. Dual-stage MZI switches are used to ensure low crosstalk with an improved signal-to-noise ratio. The delay line chip can generate a maximum delay of >1 ns with an on-chip insertion loss of 12.4 dB. Optical pulse time-division multiplexing and quasi-arbitrary waveform generation are realized based on the delay line chip. These results represent a significant step towards the realization of highly reconfigurable optical signal processors enabled by optical delay manipulation with broad applications for optical communications and microwave photonics.

Spectral hole burning and its application in microwave photonics

Ensembles of electron spins in hybrid microwave systems are powerful and versatile components for future quantum technologies. Quantum memories with high storage capacities are one such example which require long-lived states that can be addressed and manipulated coherently within the inhomogeneously broadened ensemble. This broadening is essential for true multimode memories, but induces a considerable spin dephasing and together with dissipation from a cavity interface poses a constraint on the memory's storage time. In this work we show how to overcome both of these limitations through the engineering of long-lived dark states in an ensemble of electron spins hosted by nitrogen-vacancy centres in diamond. By burning narrow spectral holes into a spin ensemble strongly coupled to a superconducting microwave cavity, we observe long-lived Rabi oscillations with high visibility and a decay rate that is a factor of forty smaller than the spin ensemble linewidth and thereby a factor of more than three below the pure cavity dissipation rate. This significant reduction lives up to the promise of hybrid devices to perform better than their individual subcomponents. To demonstrate the potential of our approach we realise the first step towards a solid-state microwave spin multiplexer by engineering multiple long-lived dark states. Our results show that we can fully access the "decoherence free" subspace in our experiment and selectively prepare protected states by spectral hole burning. This technique opens up the way for truly long-lived quantum memories, solid-state microwave frequency combs, optical to microwave quantum transducers and spin squeezed states. Our approach also paves the way for a new class of cavity QED experiments with dense spin ensembles, where dipole spin-spin interactions become important and many-body phenomena will be directly accessible on a chip.

Ultra-high Q sphere-like cavities for cascaded stimulated Brillouin lasing

High Q microsphere optical cavity is usually fabricated from a single mode fiber. Here, we propose a new method to fabricate sphere-like cavity by melting the tip of rotating quartz-rod with a CO2 laser. The cavities with diameter from 200µm to 700µm and resonant Q factors above 108 are obtained. Due to the rich resonances of the sphere-like cavity, up to 15-order cascaded stimulated Brillouin lasings(SBL) near 1.55µm are observed in a cavity with a diameter of ~760µm by simply tuning the pump wavelength to a finely-selected resonance. We wish the ultra-high Q cavities with rich resonances and bulk rod mount can have practical applications in nonlinear optics and microwave photonics as an optical component.

High-Q silicon nitride microresonators exhibiting low-power frequency comb initiation

Optical resonators with high quality factors (Qs) are promising for a variety of applications due to the enhanced nonlinearity and increased photonic density of states at resonances. In particular, frequency combs (FCs) can be generated through four-wave mixing in high-Q microresonators made from Kerr nonlinear materials such as silica, silicon nitride, magnesium fluoride, and calcium fluoride. These devices have potential for on-chip frequency metrology and high-resolution spectroscopy, high-bandwidth radiofrequency information processing, and high-data-rate telecommunications. Silicon nitride microresonators are attractive due to their compatibility with integrated circuit manufacturing; they can be cladded with silica for long-term stable yet tunable operation, and allow multiple resonators to be coupled together to achieve novel functionalities. Despite previous demonstrations of high-Q silicon nitride resonators, FC generation using silicon nitride microresonator chips still requires pump power significantly higher than those in whispering gallery mode resonators made from silica, magnesium, and calcium fluorides, which all have shown resonator Qs between 0.1 and 100 billion. Here, we report on a fabrication procedure that leads to the demonstration of “finger-shaped” Si3N4 microresonators with intrinsic Qs up to 17 million at a free spectrum range (FSR) of 24.7 GHz that are suitable for telecommunication and microwave photonics applications. The frequency comb onset power can be as low as 2.36 mW and broad, single FSR combs can be generated at a low pump power of 24 mW, both within reach of on-chip semiconductor lasers. Our demonstration is an important step toward a fully integrated on-chip FC source.

Silicon Photonics for Microwave Photonics Applications

We provide an overview of recent work on developing integrated microwave photonic subsystems in silicon photonics for generating chirped microwave waveforms as well as for providing optical true time delay. First, we describe on-chip spectral shapers based on Bragg gratings (BGs), including a distributed Fabry–Perot cavity, a Michelson interferometer incorporating identical chirped BGs, and a Sagnac loop incorporating a chirped BG, for use in wavelength-to-time mapping systems. The performance of these on-chip spectral shapers is compared to those based on microring resonators as well as fiber or free-space configurations. Second, we consider the development of an index variable optical true time delay line (OTTDL) using subwavelength grating (SWG) waveguides. For SWG waveguides of the same length, incremental delays can be obtained by tailoring the group index via control over the duty cycle. We compare the performance of SWG waveguide OTTDLs to other length variable implementations in silicon.

Invited Article: Electrically tunable silicon-based on-chip microdisk resonator for integrated microwave photonic applications

Silicon photonics with advantages of small footprint, compatibility with the mature CMOS fabrication technology, and its potential for seamless integration with electronics is making a significant difference in realizing on-chip integration of photonic systems. A microdisk resonator (MDR) with a strong capacity in trapping and storing photons is a versatile element in photonic integrated circuits. Thanks to the large index contrast, a silicon-based MDR with an ultra-compact footprint has a great potential for large-scale and high-density integrations. However, the existence of multiple whispering gallery modes (WGMs) and resonance splitting in an MDR imposes inherent limitations on its widespread applications. In addition, the waveguide structure of an MDR is incompatible with that of a lateral PN junction, which leads to the deprivation of its electrical tunability. To circumvent these limitations, in this paper we propose a novel design of a silicon-based MDR by introducing a specifically designed slab waveguide to surround the disk and the lateral sides of the bus waveguide to suppress higher-order WGMs and to support the incorporation of a lateral PN junction for electrical tunability. An MDR based on the proposed design is fabricated and its optical performance is evaluated. The fabricated MDR exhibits single-mode operation with a free spectral range of 28.85 nm. Its electrical tunability is also demonstrated and an electro-optic frequency response with a 3-dB modulation bandwidth of ∼30.5 GHz is measured. The use of the fabricated MDR for the implementation of an electrically tunable optical delay-line and a tunable fractional-order temporal photonic differentiator is demonstrated.

Silicon-on-Insulator Dual-Ring Notch Filter for Optical Sideband Suppression and Spectral Characterization

A new optical single-sideband (OSSB) modulation technique for wideband microwave photonic applications is presented. It is based on an integrated silicon-on-insulator dual-ring structure that exhibits weak electromagnetically induced transparency-like notch filter characteristics. Experimental results demonstrate optical sideband suppression of more than 29 dB for signals over 17–35 GHz with an achievable suppression as high as 40 dB. The OSSB structure is highly effective in suppressing dispersion penalties in microwave fiber transmission, with results showing a reduction in the electrical power ripple to less than ±1.5 dB for RF frequencies up to 30 GHz. The OSSB structure also enables new applications in optical vector network analyzers for optical component characterization, demonstrating the ability to measure both the amplitude and phase responses of a high-resolution ultranarrow filter based on stimulated Brillouin scattering, and also to perform wideband measurements over 10–30 GHz, offering an integrated approach for OSSB modulation that is compatible with silicon-based on-chip microwave photonic signal processing systems.

Phase-shifted Brillouin dynamic gratings using single pump phase-modulation: proof of concept.

Two novel phase-shifted Brillouin dynamic gratings (PS-BDGs) are proposed using single pump phase-modulation (SPPM) in a polarization maintaining fiber (PMF) for the first time to our knowledge. Firstly, based on the stimulated Brillouin scattering (SBS), a transient PS-BDG with a 3-dB bandwidth of 354MHz is written by a 2-ns pump1 pulse and a 100-ps pump2 pulse, where the phase of pump1 pulse is shifted with π from its middle point through phase modulation. Then, with a high repetition rate of 250MHz for both pump pulses, an enhanced PS-BDG with a deep notch depth is obtained and its notch frequency can be easily tuned by changing the phase shift. We demonstrate a proof-of-concept experiment of the transient PS-BDG and show the notch frequency changing by tuning the phase shift. The proposed PS-BDGs have important potential applications in microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

Femtosecond Laser-Based Microwave Signal Generation and Distribution

We review our most recent progress in microwave photonic applications of ultralow timing jitter femtosecond mode-locked fiber lasers. Sub-femtosecond timing jitter (integrated from 10 kHz to >10 MHz offset frequency) optical pulse trains can be generated from various types of mode-locked fiber lasers using dispersion engineering and intracavity filtering. To fully utilize such ultralow-jitter lasers for microwave photonic applications, we demonstrate a sub-femtosecond-resolution (−159 dBc/Hz phase noise floor) phase detection method between optical pulse trains and microwave signals using a Sagnac-fiber-loop-based device named the fiber-loop optical-microwave phase detector (FLOM-PD). Using ultralow-jitter mode-locked Er-fiber lasers and FLOM-PDs, we generate 10-GHz microwave signals with −142 dBc/Hz absolute single-sideband phase noise at 10-kHz offset frequency. When an all-fiber Michelson interferometer-based repetition-rate stabilization method is further employed, the phase noise is suppressed to −90 dBc/Hz at 10-Hz offset frequency, which results in 3-fs absolute rms timing jitter integrated from 10 Hz to 10 MHz offset frequency. Long-distance microwave phase transfer via optical fiber links is also demonstrated using the FLOM-PD as a means for stabilizing pulse time-of-flight in fiber transfer. Relative frequency instability of 6.5 × 10−19 is demonstrated for 2.856-GHz microwave signals transferred over a 2.3-km-long fiber link. We anticipate that the capability of generation, characterization, stabilization, and transfer of ultralow-noise microwave signals using ultralow-jitter femtosecond mode-locked lasers will find more applications in microwave photonics in the near future.

Microwave photonics connected with microresonator frequency combs

Microresonator frequency combs (micro-combs) are very promising as ultra-compact broadband sources for microwave photonic applications. Conversely, microwave photonic techniques are also employed intensely in the study of microcombs to reveal and control the comb formation dynamics. In this paper, we reviewed the microwave photonic techniques and applications that are connected with microcombs. The future research directions of microcomb-based microwave photonics were also discussed.

GaAs-based polarization modulators for microwave photonic applications

GaAs-based polarization modulators (PolMs) exhibit the unique characteristic of simultaneous intensity and complementary phase modulation owing to the linear electro-optic (LEO) effect determined by crystallographic orientations of the device. In this paper, we reviewed the principle of operation, the design and fabrication flows of a GaAs-based PolM. Analytical models are established, from which the features of a PolM are derived and discussed in detail. The recent advances in PolM-based multifunctional systems, in particular the PolM-based optoelectronic oscillator (OEO) are demonstrated with an emphasis on the remarkable development of applications for frequency conversion, tunable microwave photonic filter (MPF), optical frequency comb (OFC), arbitrary waveform generation (AWG) and beamforming. Challenges in practical implementation of the PolM-based systems and their promising future are discussed as well.

Dual-Mode Semiconductor Lasers and Their Applications in Optical Clock Recovery and Photonic Microwave Generation

As a compact multi-functional device, monolithic integrated multi-section semiconductor laser has a wide range of applications in optical communication systems and microwave photonics with its advantages in cost, size, power consumption and integration possibilities with other components. In this paper, we will review the design and characteristics of monolithically integrated dual mode semiconductor laser, with special emphasis on amplified feedback lasers (AFL). Then, their application in optical clock recovery and high frequency photonic microwave generation will be discussed

Stimulated Brillouin scattering gain bandwidth reduction and applications in microwave photonics and optical signal processing

Stimulated Brillouin scattering (SBS) is one of the most dominant nonlinear effects in standard single-mode fibers and its unique spectral characteristics, especially the narrow bandwidth, enable many different applications. Most of the applications would benefit from a narrower bandwidth. Different methods for the bandwidth reduction of SBS in optical fibers are presented and discussed. A bandwidth reduction down to 17% of the natural gain can be achieved by the superposition of the gain with two losses or the utilization of a multistage system. Furthermore, applications in the field of microwave photonics and optical signal processing like high-resolution spectroscopy of communication signals, the storage of optical data packets as well as the processing of frequency combs including generation of millimeter waves and ideal sinc-shaped Nyquist pulses are presented.

Tuning of resonance spacing over whole free spectral range based on Autler-Townes splitting in a single microring resonator

In this paper, a single microring resonator structure formed by incorporating a reflectivity-tunable loop mirror is demonstrated for the tuning of resonance spacing. Autler-Townes splitting in the resonator is utilized to tune the spacing between two adjacent resonances by controlling the strength of coupling between the two counter-propagating degenerate modes in the microring resonator. A theoretical model based on the transfer matrix method is built to analyze the device. The theoretical analysis indicates that the resonance spacing can be tuned from zero to one free spectral range (FSR). In experiment, by integrating metallic microheater, the tuning of resonance spacing in the range of the whole FSR (1.17 nm) is achieved within 9.82 mW heating power dissipation. The device has potential for applications in reconfigurable optical filtering and microwave photonics.

Brillouin Rectangular Optical Filter With Improved Selectivity and Noise Performance

We demonstrate a high selectivity rectangular optical filter based on stimulated Brillouin scattering (SBS) effect in fiber with bandwidth from 1 to 3 GHz. By using pump-splitting dual-stage configuration, the filter selectivity has been improved by more than 16 dB and exceeded 40 dB within 2-GHz bandwidth. The filter shape is well controlled by using precise digital feedback compensation and nonlinearity management. We also analyze the noise performance of both the single-stage and dual-stage filters by evaluating the signal-to-noise ratio of the amplified signal using coherent detection. The results show that the dual-stage approach performs better than the single-stage architecture in achieving higher selectivity and improving noise performance. The inherent tuning flexibility with improved performance makes the proposed SBS filter one of the best candidates for microwave photonic applications.

Dual-function photonic integrated circuit for frequency octo-tupling or single-side-band modulation.

A dual-function photonic integrated circuit for microwave photonic applications is proposed. The circuit consists of four linear electro-optic phase modulators connected optically in parallel within a generalized Mach-Zehnder interferometer architecture. The photonic circuit is arranged to have two separate output ports. A first port provides frequency up-conversion of a microwave signal from the electrical to the optical domain; equivalently single-side-band modulation. A second port provides tunable millimeter wave carriers by frequency octo-tupling of an appropriate amplitude RF carrier. The circuit exploits the intrinsic relative phases between the ports of multi-mode interference couplers to provide substantially all the static optical phases needed. The operation of the proposed dual-function photonic integrated circuit is verified by computer simulations. The performance of the frequency octo-tupling and up-conversion functions is analyzed in terms of the electrical signal to harmonic distortion ratio and the optical single side band to unwanted harmonics ratio, respectively.

Design of heterogeneous multicore fibers as sampled true-time delay lines.

We present a novel procedure for designing a sampled discrete true-time delay line (TTDL) for Microwave Photonics applications based on a heterogeneous MCF. Both simple step-index (SI) and trench-assisted SI profiles are numerically evaluated in terms of physical dimensions and material dopant concentrations in order to individually tailor the group delay and chromatic dispersion of each core. The proposed TTDL features unique properties beyond the current state of the art in terms of record bandwidth, compactness, flexibility, and versatility.

Dispersive Fourier Transformation for Versatile Microwave Photonics Applications

Dispersive Fourier transformation (DFT) maps the broadband spectrum of an ultrashort optical pulse into a time stretched waveform with its intensity profile mirroring the spectrum using chromatic dispersion. Owing to its capability of continuous pulse-by-pulse spectroscopic measurement and manipulation, DFT has become an emerging technique for ultrafast signal generation and processing, and high-throughput real-time measurements, where the speed of traditional optical instruments falls short. In this paper, the principle and implementation methods of DFT are first introduced and the recent development in employing DFT technique for widespread microwave photonics applications are presented, with emphasis on real-time spectroscopy, microwave arbitrary waveform generation, and microwave spectrum sensing. Finally, possible future research directions for DFT-based microwave photonics techniques are discussed as well.

Cascaded Brillouin lasing in monolithic barium fluoride whispering gallery mode resonators

We report the observation of stimulated Brillouin scattering and lasing at 1550 nm in barium fluoride (BaF2) crystal. Brillouin lasing was achieved with ultra-high quality (Q) factor monolithic whispering gallery mode mm-size disk resonators. Overmoded resonators were specifically used to provide cavity resonances for both the pump and all Brillouin Stokes waves. Single and multiple Brillouin Stokes radiations with frequency shift ranging from 8.2 GHz up to 49 GHz have been generated through cascaded Brillouin lasing. BaF2 resonator-based Brillouin lasing can find potential applications for high-coherence lasers and microwave photonics.